1. Field of the Invention
The present invention generally relates to passive infrared detection systems and, more particularly, is concerned with a composite Fresnel lens having an array of Fresnel lens segments providing a long narrow detection range that adapts the lens for use in a detection system for covering a long narrow area of a building such as a hallway.
2. Description of the Prior Art
A passive infrared detection system detects heat energy radiated or emitted by an object, such as the body of a person, moving across the field of view of a heat sensor, such as a pyroelectric detector, of the detection system. The detection system must be capable of distinguishing between background temperature and the moving object having a different temperature from the background. Thus, the detection system has an electrical circuit operatively coupled to the heat sensor for producing a detection signal in response to the heat sensor detecting a change of temperature as, for example, caused by the body heat of a person entering the detection pattern. The sensitive area of the heat sensor is too small to detect a significant amount of heat energy radiated from a human body at a distance by using the heat sensor alone. Thus, detection systems typically employ a lens array made up of Fresnel lens segments for collecting and focusing a significantly greater amount of heat energy from specific distant areas on the heat sensor than would be received using the heat sensor alone.
A Fresnel lens segment is most efficient when it is flat, however, typically a lens array of Fresnel lens segments must be provided in a curved configuration in order to collect and focus the desired amount of heat energy on the heat sensor from a wide angle detection pattern. Thus, the lens array of Fresnel lens segments approximates this flatness by being provided as close-to-flat vertical segments arranged in contiguous side-by-side relationship along a curve having a constant radius. As known by those skilled in the art, such flat vertical segments are produced from a complete, whole Fresnel lens having a given diameter and a given focal length, and accordingly, the resultant lens segment will maintain the same characteristics as the whole lens from which it was taken. As used herein, the Fresnel lens from which the segment was taken will be referred to hereinafter as the original whole Fresnel lens.
The configuration of the Fresnel lens segments and their arrangement in a lens array will depend on the shape of the area from which energy is to be collected by the lens array. For example, a lens array to cover the area of a long narrow hallway in a building will need to utilize Fresnel lens segments with configurations and in an arrangement substantially different from those of a lens array to cover a relatively square or rectangular room in the building.
A factor complicating the design of Fresnel lens segment arrays for coverage of different shaped areas is the economic requirement that these arrays must be interchangeable so as to be able to fit and mount within a common housing allowing such interchangeability either at the factory or in the field. This requirement means that the size and the curved configuration of a lens array designed to cover a hallway be of the same size and have the same radius of curvature as a lens array designed to cover a room.
Consequently, a need exists for a design of a hallway lens array that will efficiently and effectively cover the long narrow area of a hallway while still being interchangeable with one covering a room so that the lens arrays can use a common housing.